tools: protoxform support for oneof field promotion. (#9520)
tools: protoxform support for oneof field promotion. (#9520)

Inspired (but no longer required) by #9500. This is a relatively simple
API migration feature to support, so worth adding for future flexibility

Risk level: Low
Testing: New protoxform golden tests added.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 4efbf45c10e2d2afd84fee2466e476cb8fc0a804

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api